FIG. 1 of the accompanying drawings shows a cam summation engine valve system as disclosed in U.S. Pat. No. 6,941,910, upon which the present invention is based. The valve system comprises two cams 10 and 12 and a cam summation rocker 14, herein also termed an upper rocker, having cam followers 16 and 18 in contact with both cams. A lower actuating rocker 20 pivotably connected to the summation rocker 14 acts at one end on a valve 22 and rests at its other end on a hydraulic lash adjuster 24. An adjustable stop plate 26 is used to limit the expansion of the hydraulic lash adjuster 24 by setting the height of the pivot shaft 30 that connects the lower rocker 20 to the upper rocker 14. The position of the lower rocker 20 is therefore defined by its contact with the tip of the valve 22, and the expansion of the hydraulic lash adjuster 24 holding the pivot shaft 30 against the adjustable stop plate 26.
Cam summation valve systems using hydraulic lash adjusters have required an adjustable stop, or a graded shim in order for the system clearance (and hence the valve lift) to be adjusted. The functions of this clearance adjustment are twofold. First, the expansion of the hydraulic lash adjusters is limited so that the correct amount of clearance is maintained in the system whilst the valves are closed. Second, the valve actuating rocker is held in contact with the tip of the valve by the expansion of the hydraulic lash adjusters and the clearance adjustment system so that any clearance must occur between one of the cam profiles and its respective follower(s).
The Applicants have earlier proposed in WO2008/139221 a cam summation engine valve system as shown in FIG. 2 of the accompanying drawings. This figure shows a similar valve system to that shown in FIG. 1 and in it like parts have been allocated the same reference numerals to avoid repetition. Here, the valve actuating lower rocker 20 is mounted on a manually adjustable pivot 32. The valve lift is adjustable by means of a screw mechanism 34 and contact is maintained between the tip of the valve 22 and the lower rocker 20 at all times by means of a control spring 36.
This design does not benefit from an automatic lash adjuster, which is instead replaced by a mechanical clearance adjustment that maintains the correct amount of clearance in the system whilst the valves are closed. However, an adjustable pivot 32 is required to allow the amount of clearance in the system to be adjusted. In the absence of such adjustability, there would be no way to compensate for manufacturing tolerances, which may lead to significant variations in valve lift between cylinders, and potentially damaging impact forces between the components of the system. While it provides for clearance adjustment, the system of WO2008/139221 requires a significant amount of packaging space that may not be available in all engines. Furthermore the use of a hydraulic lash adjuster can compensate for the effects of thermal expansion and component wear throughout the life of the engine whilst a mechanical adjustment has to suit all operating conditions throughout the life of the engine.